I. Field of the Invention
This invention relates generally to fluid handling pumps, and more particularly to an improved design of a rotary peristaltic pump especially adapted for metering measured quantities of various fluids, especially abrasive fluids or viscous fluids.
II. Discussion of the Prior Art
A variety of pump types have been used in the past for pumping liquids which are abrasive, viscous or that must be pumped in precisely metered quantities. Typically, piston pumps and gear pumps are unsuitable in such applications. When dealing with abrasive fluids, piston pumps and gear pumps are subject to rapid wear and failure. Progressive cavity pumps, diaphragm pumps, hose-type peristaltic pumps and open bladed rubber impeller pumps, while suitable for pumping abrasive liquids suffer from other drawbacks relating to size, cost, viscosity, mounting, chemical compatibility and wear do not make these pumps suitable for all types of metering applications.
For example, progressive cavity pumps incorporating a helical screw have been used for many years to handle abrasive and viscous fluids. While they exhibit minimal pulsation in the flow of liquid being delivered, they tend to be costly, overly long for certain applications and generally require a double universal joint on a connecting rod located in the fluid being pumped to follow eccentric motion of the screw rotor. Also, vibration of the pump due to eccentric motion of the rotor must be addressed. The progressive cavity pumps have found application primarily in aggressive industrial fluid delivery systems where space, cost and mounting requirements are not restrictive.
Diaphragm pumps are also commonly used for delivering abrasive and viscous liquids. While relatively cost effective and small in size, they do require valves which are subject to failure and require frequent replacement. Further, the fluid flow necessarily pulsates as the diaphragms are reciprocally displaced. Because of the pulsatile flow, diaphragm pumps are most often used for liquid transfer rather than metered flow.
Peristaltic pumps are most commonly of the tube (hose) variety where the tube is periodically squeezed of fluid by means of rollers that are made to rotate about a central shaft. Peristaltic pumps offer the advantage of simplicity and ease of maintenance, as it is only necessary to change a hose or tube while retaining the motor-driven roller assembly used to squeeze the fluid in the hose and thereby advance it towards a pump outlet. Such pumps are void of valves and are capable of pumping a wide variety of fluids.
The drawback of conventional peristaltic pumps is that the tube must recover its cylindrical shape following the passage of each roller there over and the time required for the tube's elasticity to restore it to its normal shape necessarily limits the operating speed of such a pump. Also, such pumps inherently create a pulsatile flow.
Conventional prior art rubber impeller pumps have been noted for their ability to pump fluid with no pulsation and also to handle abrasive fluids. However, they do suffer the disadvantage of low pressure operation and high amounts of internal slip, particularly under relatively modest output pressures. Further, the open blade impeller material must have mechanical properties which allows the blade to follow the profile of the housing. This restricts the material selection almost exclusively to elastomer such as neoprene, Buna-N and EPDM. Thus, they are typically used in low pressure fluid transfer and circulation. Typically, rubber impeller pumps are used in household washing machines to empty water from its drum into a drain or as a water pump used in the cooling systems for internal combustion engines.
In certain metering applications, such as delivery of precise quantities of colorants to a white tint base in preparing different shades and tints of paint used in decorating or the like, a microprocessor-based controller is typically used to drive a stepper or servo motor that, in turn, drives a metering pump for depositing a selected volume of a selected colorant into the white tint base. For the reasons advanced above, prior art pumps of the type described are less than adequate in this application. First of all, it is recognized that paint colorants are generally highly abrasive and would rather quickly wear out a piston or gear-type metering pump. It is difficult to obtain the desired precision from pumps exhibiting pulsatile flow. The internal slip of traditional rubber impeller pumps also results in variation in the quantity of material delivered per revolution or fraction thereof and make that type of pump unsuitable for precision applications.
Thus, a need exists for a pump capable of handling abrasive and viscous fluids, that does not require valves and that is capable of pulsation-free operation. It is the object of the present invention to provide just such an arrangement.